A pedigreed colony of the Chilean hystricognathous rodent, Octodon degus, has been established. The animals are maintained in a closed, random-bred colony; in addition, an inbreeding program is in progress to derive isogeneic degus. The degu is a rat-like rodent reaching and adult weight of approx. 300 grams. The coat color is agouti; a coat color variant has been identified. This species belongs to the porcupine suborder, and is related at the subordinal level to the guinea pig. Gestation time is 90 days, with two apparently seasonal breeding periods per year. The animal is docile and thrives in conventional vivarium facilities with no extraordinary care required. The degu has an anatomically separate dual thymus system: a large, bilateral cervical thymus lying caudal to the submaxillary salivary gland in the neck; and a mediastinal thymus. Spontaneous bilateral cataracts arise with considerable frequency. Immunological studies include: 1) the effect of cervical thymectomy on allogeneic transplants in newborn degus; enumeration of antigen binding cells in cervical and mediastinal thymus, peripheral lymphoid tissues and thoracic duct cells using horse radish peroxidase and beta-galactosidase as antigen; the effects of immunization and thymectomy on numbers of antigen binding cells in lymphoid populations.